The present invention relates to a communication apparatus which uses an Asynchronous Transfer Mode (ATM) communication system, and an exchange or terminal equipment for terminating a line or a connection in an ATM network, and more particularly to a failure detection and notification apparatus provided at a line interface of such apparatus.
As disclosed in an article "Virtual Path OAM Functions for Broad-band ATM Networks", Kanayama et al, Technical Journal of Society of Electronic Engineering and Communication, CS91-92, when a transmission line failure occurs in an ATM network, all Virtual Paths (VPs) on the transmission line fail. As a result, it is necessary to make a failure notification to all users of the failed VPs (ATM exchanges, terminal equipments) to rapidly conduct the stop of charge in the exchanges, the disabling of the lines and the switching at the VP level.
Since all information are transferred by cells on the VP, a previously defined alarm indication cell is transferred to notify of the occurrence of a failure.
An ATM transfer device such as a cross connect (XC) has a VP routing table (VPI (Virtual Path Identifier) conversion table) for VPs accommodated in each transmission line, and a failure VP number can be easily identified by using the table so that an alarm indication cell can be rapidly transferred to the failed VP.
Referring to FIG. 7 which is quoted from the above-mentioned article, an example of a sequence of an alarm transfer is explained. In FIG. 7, numeral 301 denotes an ATM-SW (ATM exchange), numeral 302 denotes an XC-a (cross connect), numeral 303 denotes an XC-b, numeral 304 denotes an XC-c, numeral 305 denotes an other ATM-SW, numeral 306 denotes a VP alarm indication signal (AIS) cell, numeral 307 denotes a VP-far end receive failure (FERF) cell, numeral 308 denotes a transmission line failure and numeral 309 denotesan S-FERF (Section-FERF).
When the XC-b 303 detects the transmission line failure 308, it sends out the section failure signal S-FERF 309 to the XC-a 302, and determines the VPs accommodated in the failed transmission line using the VP routing table. The VPI number of the failed VP is set in the VPI field of the VP-AIS (Virtual Path-Alarm Indication Signal) cell 306 so that the VP-AIS cell 306 is routed in the network as is user cells and transferred to the end point. Information, a type of failure and a location of failure, is written into a payload of the VP-AIS cell 306 by the failure detection circuit of XC-b 303.
Thus, when the ATM-SW 305 detects the VP-AIS cell 306, it can localize the failure. When the end point device ATM-SW 305 receives the VP-AIS cell 306, it sends out the alarm indication cell (VP-FERF cell 307) which notifies the failure of the VP in the sending side so that the failure of the sending VP may be detected at a far end point of VP, the ATM-SW 301.
In the detection of the VP-AIS state, rapidness and sufficient reliability to the failure detection are required. In order to prevent the VP-AIS cells from disturbing the transfer of user cells, the bandwidth of the VP-AIS cells to be transmitted is limited and the VP-AIS cells are periodically sent out. This is defined in the CCITT Recommendation (Recommendation I.610:B-ISDN Operation and Maintenance Principles and Functions).
The VP of the communication apparatus shifts the state as shown in a state transition diagram of FIG. 8 when it receives the VP-AIS cell. Numeral 401 denotes a normal state of the VP and numeral 402 denotes the VP-AIS state. The number of VP-AIS cells required for the failure detection is "1". Thus, in the normal state, by receiving one VP-AIS cell (VP-AIS cell reception 404), the state is changed to the VP-AIS state 402.
FIG. 9A shows a case of a transmission line failure in other apparatus. The ATM-SW 305 operates in the following manner. (1) It detects the VP-AIS cell 306 sent from other communication apparatus (e.g. XC-b 303 of FIG. 7) (2) It sets the VP having detected the VP-AIS cell thereof to "VP-AIS state" in accordance with the state transition diagram shown in FIG. 8. (3) It prepares the VP-FERF cell 307 and sends it to the far end VP end point.
FIG. 9B shows a case of a transmission line failure in its own apparatus. The ATM-SW 305 operates in the following manner. (1) It detects the transmission line failure. The transmission line interface in the ATM-SW has a function to monitor the power of the signals and the synchronization status of the signals, and when it detects a drop in the signal power or a out-of-synchronization status, it sends out a failure signal to the failure detection unit of the ATM-SW through a separate signal line. (2) It determines the VPs accommodated in the failure transmission line using the routing table in the same manner as the XC-b 303 detects the transmission line failure. It sets all VPs accommodated in the failure transmission line to the "VP-AIS state" as long as the transmission line stays in the failure state. (3) When the VP is terminated at the ATM-SW, it prepares the VP-FERF cell 307 and sends it to the far end VP.
FIG. 9C shows a case where the transmission line failure is in its own apparatus and the failure transmission line 312 accommodates the VP 310 which is terminated by the ATM-SW and the VP 311 which is not terminated by the ATM-SW. The state transition of the failure detection is same as (1) and (2) of FIG. 9B. The ATM-SW 305 prepares the VP-AIS cell 306 for the non-terminated VP 311 and sends it downward. On the other hand, it prepares the VP-FERF cell 307 for the terminated VP 310 as is the case of FIG. 9B and sends it to the far end VP end point.
As shown in FIGS. 9A, 9B and 9C, there are two cases for the operation of the ATM-SW. Namely, (1) It detects the failure state of the VPs accommodated in the transmission line of its own apparatus when it fails. (2) It detects the failure state of the VP having the VP-AIS cell thereof sent out when the transmission line of another apparatus fails.
In the case (1), the transmission line (section) failure is signaled from the transmission line interface through the separate signal line, and in the case (2), the transmission line failure is signaled by the VP-AIS cell. Thus, two different signals are inputted to set the VP to the same failure state.
Further, if a transmission line failure occurs in the transmission line of its own apparatus when a VP is in the VP-AIS state in the state transition diagram of FIG. 8 by the transmission line failure of the other apparatus, no external cell is inputted because of the transmission line failure of its own apparatus and hence no VP-AIS cell is inputted. In this case, "VP-AIS cell not received for 3 seconds" 406 is indicated and the state is shifted to the normal state in accordance with the state transition diagram of FIG. 8. Since the transmission line failure has occurred in its own apparatus, the corresponding VPs should be kept in the failure state but in this case, the VP-AIS state is released.